Bag making machines wherein a heat sealable film is advanced over a forming shoulder and along a forming tube, and transversely sealed at spaced intervals to form bags have been known heretofore, but either the transverse sealing jaws have been used to advance the film, or other means have been used to advance the film while the sealing jaws remain stationary and do not move axially of the forming tube.
The use of separate means to advance the film has the advantage of relieving tension on the film, which is otherwise present when the sealing jaws are also used to advance the film. Furthermove, some films have a tendency to stretch if the sealing jaws are used for advancing the film. This, obviously, is a disadvantage from the standpoint of uniformity in the length of the bags being formed.
When means are used to advance the film which are independent of the sealing jaws, and when the jaws also move with the film while the film is advancing, but which do not themselves cause the film to advance, there results an elimination of film tension above the sealing jaws. This relaxed condition of the film allows low density product to more readily settle into the bottom of the bag. This is particularly true with respect to a pillow bag. This arrangement is further advantageous, however, in forming a flat bottom bag, since the relaxed condition of the film allows the flat bottom, which was formed at the top of the stroke, to be maintained while the film is being advanced and filled.
Also, flat bottom bags have been made heretofore, on machines which have used the principle of advancing the film by means of the sealing jaws, as shown for example in U.S. Pat. No. 3,552,081, and therefore were subject to the disadvantages mentioned above.
Friction belt means have also been used heretofore for advancing the film, such as shown, for example, in U.S. Pat. No. 3,826,061, but the sealing jaws did not move with the advancing film, and, therefore, this arrangement was likewise subject to the disadvantages referred to above.
These disadvantages have been overcome in the present invention, which enables the manufacture of flat bottom bags more rapidly than has heretofore been possible.